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Thread: Sawstop
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20th December 2020, 05:25 PM #16
Interesting that it doesn't use a disk brake like arrangement with a loaded heavy spring to drop/pull the armature down/away.
It would save the blade and cost to re-arm would be zero.
edit - An electro-magnetic brake would do the same job. The ones metal workers use are simply ferocious (haha! that pun is awful!).
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20th December 2020, 08:20 PM #17
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20th December 2020, 10:49 PM #18GOLD MEMBER
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21st December 2020, 09:22 AM #19
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21st December 2020, 09:27 AM #20Mobyturns
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21st December 2020, 10:33 AM #21.
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Can't be be any worse than a motor vehicles disc brake exposed to road dust/grit mud etc.
One problem with fitting a disc brake to a table saw spindle is that this would inhibits the depth of the cut as the disc has to be kept under the table otherwise it will have to poke above the table and interfere with the work. This is not a problem in the case of a lathe or other machinery which can have a dis mounted at the other end of the spindle without interfering with the work.
The disc could be mounted off axis eg on the motor, but then the system is using the belts to stop the blade which would not be as instantaneous as a caliper direct on a blade.
FWIW I have a home made foot operated disc brake on my bandsaw motor. The brake pads are from a Colt Galant, the caliber is from hospital gurney and the rest is home made.
With the 6tpi 25mm Blade on the saw I can comfortably stop the blade in about 3s - I could stop it faster (eg <2s) but I worry about the stresses involved. For a BS it would be better to have the disc on one of the wheels.
Painted-Brake.jpg
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21st December 2020, 12:11 PM #22
No more so than the rotations from a little disk brake on a mountain bike.
I'm a Chunky Lad and love biking. The disk brakes when they first came out looked too tiny to be effective, but they pull up my mighty frame and bike quick smart, even down hill at great velocities (arguably the very best time for such up-pulling).
A 12" saw blade at 6000 rpm is NOTHING compared to the masses I just described
An electric disk brake would clamp that saw blade to a dead stop absolutely instantaneously.
On an aside, I've been seeing more electric disk brakes everywhere recently - trailers, caravans, those little tents-in-a-trailer, cars (drive by wire!) and in the Formula 1 they are absolute.
Seems to me that a retrofit kit for ordinary tablesaws should be dead easy from an engineering perspective.
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21st December 2020, 03:42 PM #23.
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Here are some interesting BOTE calcs
Kinetic energy of 120kg person+bike at 60kph = 15360 joules.
To stop that bike rider combo in 1s requires a power dissipation rate of 15.36kW
According to several web sources in practice the best a pro rider can stop in and still stay on a bike is between 1.5 and 2 secs so the power dissipation required is about half that above.
Mind you that will NOT be for 120kg bike+rider - ie more like a 70kg cyclist and 10kg bike so lets half that again.
The KE of 300mm saw blade + blade Pulley + motor pulley + motor spindle all travelling at 6000 rpm is only about 100J
BUT
If you want to stop the saw blade + blade pulley + motor pulley + motor spindle, in 5ms requires the ability to dissipate 20kW over that time!
Not saying its not possible (i'd even be surprised if if it can't be done) but its not as straight forward as it may appear.
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21st December 2020, 10:09 PM #24
Ah, it is very different!
A motor vehicles disk brakes are designed to slow / stop a vehicle in an acceptable period of many seconds and is not designed to prevent all / any injury. There are other safety systems in a modern vehicle that provide a "safety system" - seat belts, air bags, side intrusion bars etc.
The SawStop scenario is to "instantly" stop the saw blade, and retract it into the table to prevent any injury. It is the only safety mechanism in place, so must deploy with 100% reliability to be effective. If a "disk brake" mechanism is to perform the same functions in the same millisecond time span then it would want to be a pretty well designed unit like nothing currently available.Mobyturns
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22nd December 2020, 12:16 AM #25.
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Not quite. A motor vehicles disc brake system is designed to slow/stop etc. . . . . .
The Disc brakes themselves are designed to stop the wheels ASAP - it's the auxiliary components like ABS that releases brake pressure and just keeps the wheels turning to reduce skidding and maintain steerage/control etc
A modern motor vehicle disc brake can instantly lock a rotating vehicle wheel if no ABS is present and there's no forward vehicle momentum OR vehicle weight to generate wheel/road friction that keeps the wheel turning. Just think about a car with no ABS driving on ice. A stomp on the disc brake will easily lock the wheels.
Now replace the pedal with a 1ms relay to cut the motor mains and activate an EM actuator at the calliper. BANG ! It's not going to be kind to wheel bearings and shafts etc if this is the only way to stop the saw but the same applies to the saw stop. These devices are meant for emergencies not a regular way to stop rotation.
BUT going back to what I was referring to in terms of the dirt and dust on the rotor and pads etc this makes very little difference - just look at rally cars etc on those dusty rods etc.
FWIW a 750kg F1 car at 200 kph can stop in about 3s during which each wheel must generate of ~100 kW of braking power - their brakes can deliver away more stopping power than this but just like an regular vehicle they are no good if they lock up - which is what we want to do for a TS.
20kW for a saw blade over 5ms starting to look doable but I suspect more expensive than a bit of honeycomb Al and an explosive bolt.
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22nd December 2020, 12:59 PM #26
TRex teeth....
I was also thinking about the rotation of the blade in all of this.
A 12" (300mm) blade at 6000 rpm is moving at ~ 95 m/s .... a fair clip! (30pi=94.3cm x 100)
If a saw takes one millisecond to detect, fire and engage to an utter stop, that blade has moved 9.5cm
That's a lot of chomping on a finger. 10cm of blade has a few teeth in it.
Hence the pulling down/away mechanism.
It all makes sense now why its designed the way it is. Stopping it alone is not enough.
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22nd December 2020, 03:07 PM #27
the bit I've highlighted in bold italic is as I understand it is the basis of Steve Glass's patent.
"anyone" can drop and stop a saw blade in 100 millionths of a second -- the energy involved is not that great.
the key technology that Glass patented is the 200 kHz pulsing detector. As I understand the patent dispute with the Bosch Reaxx saw, the Reaxx used the same or sufficiently similar technology to breach SawStop's patents.regards from Alberta, Canada
ian
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22nd December 2020, 03:13 PM #28
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22nd December 2020, 05:05 PM #29rrich Guest
I read both of the Steve Gass patents.
IIRC, the basis of both patents is sensing flesh coming into contact with the blade removes the blade from contact with flesh.
When I read that, I realized that the patent must expire before there will be competition.
However, the system mentioned here is flesh near the blade activates the mechanism. So maybe this system will be practical. As far as sitting on a jury for patent infringement and the word "contact" could be the deciding factor. I know that if the Gass system requires blade contact and this system does not, IMNSHO there is no infringement. But Gass is a lawyer with resources. A lengthy court battle could easily bankrupt a small start up company.
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26th December 2020, 11:51 PM #30Member
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Same response!
I got the same answer, they assured me that it fired due to contacting metal.... I know i can't change anyones mind and it is what it is, bt there was no metal anywhere near the blade and the the timber was clean..... I wanted to find what I did wrong to ensure I do not repeat the issue. I know for sure I did not make contact with metal.....so now I am second guessing myself every time I run a bit of timber through just waiting for the next bang! Shame, the saw otherwise is brilliant.WIGWOOD https://www.medalboxes.com.au
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